The invention relates to a filter structure comprised of coaxial resonators, which structure is especially applicable as an antenna filter for base stations of radio networks.
The requirements imposed upon a radio-frequency filter of a base station are relatively strict regarding e.g. the width of the transition band between the pass band and stop band as well as the stop band attenuation. Therefore, the order of the filter tends to be rather high. For the resonator filter this means that the structure will comprise several individual resonators and coupling arrangements therebetween. On the other hand, the attenuation of the filter shall be low in the pass band, which limits the number of resonators as well as their losses.
There are several known filter structures based on resonators. Resonators are usually arranged in one or two rows so that they constitute a metal casing which appears as a single block when viewed from the exterior. The most common resonator type is the coaxial quarter-wave resonator. Inter-resonator coupling, which is accomplished by means of auxiliary parts, is either capacitive or inductive. Coupling mechanism details may vary to a great extent. FIG. 1 shows an example of such a prior-art filter partly cut open. It comprises a total of six coaxial resonators the cavities of which are formed so that the space of the metal casing of the filter is divided by one longitudinal and two transversal partition walls into two three-cavity rows. The first row comprises the first 110, second 120, and the third 130 resonator. The second row comprises the fourth 140, fifth 150, and the sixth 160 resonator so that the sixth resonator is side by side with the first resonator. Couplings in the filter are such that the signal is brought into the first resonator 110 and it travels a U-shaped path via the second, third, fourth and fifth resonators into the sixth resonator 160 where it goes out. Each resonator comprises an inner conductor, such as 131 and 141, depicted vertical in FIG. 1, and a horizontal planar extension to the inner conductor, such as extensions 132 and 142. The extension adds to the capacitance in the upper end, or the open end, of the structure, thereby the resonator can be made shorter in the vertical dimension. Each resonator further comprises an outer conductor comprised of parts of the resonator partition walls, side walls of the whole filter case, and end walls in some resonators. The structure is a quarter-wave resonator because each inner conductor is by its lower end connected to a conductive bottom plate 101 which is part of the signal ground. The line comprised of the inner conductor and outer conductor is thus short-circuited at its lower end. The structure includes a conductive cover 102 so that the filter case is closed. FIG. 1 shows some of the inter-resonator couplings. On the same height with the extensions of inner conductors there are two apertures 116, 117 in the partition wall between the first and the second resonator so that the said resonators are capacitively coupled through the said apertures. A similar aperture 156 can be seen in the partition wall between the fifth and the sixth resonator. In the partition wall between the second and the third resonator there are similar apertures which cannot be seen in FIG. 1 because the partition wall has been cut out. Instead the Figure shows two vertical projections 123, 124 in the extension 122 of the inner conductor of the second resonator, placed so as to face the apertures in the partition wall between the second and the third resonator, thereby adding to the strength of capacitive coupling. A similar vertical projection 133 is found in the extension of the inner conductor of the third resonator, facing the aperture (not shown) in the partition wall between the third and the fourth resonator. Moreover, there is inductive coupling between the third and the fourth resonator. This is realized by means of conductive projections 134, 144 at the lower ends of the inner conductors 131, 141 and an aperture in the lower part of the partition wall 107.
Filters like the one depicted in FIG. 1 are often realized as Chebyshev filters because this structure is the most convenient for producing the required narrow transition band on one side of the pass band. On the other hand, Chebyshev approximation means that there will appear pass band attenuation variation in the amplitude response of the filter. To reduce the pass band attenuation variation, one needs to increase the order of the filter and, thus, increase the number of resonators. More resonators in the signal path may in turn raise the basic pass band attenuation too high.
An object of the invention is to alleviate the said disadvantage associated with the prior art. The structure according to the invention is characterized by that which is specified in the independent claim 1. Some preferred embodiments of the invention are specified in the other claims.
The basic idea of the invention is as follows: on the side of a resonator chain constituting a bandpass filter there is provided an additional equalizing resonator, coupled to a resonator in the chain. The resonance frequency of the equalizing resonator and its coupling to the rest of the filter are arranged so that the transfer function of the filter gets a new zero at a point corresponding to an attenuation minimum. Thereby the attenuation at that point increases with the result that attenuation variation in the whole pass band decreases. The Q factor of the equalizing resonator is arranged to be so small that the arrangement increases filter attenuation over a relatively wide range within the pass band. Response equalization can be further enhanced by providing a second equalizing resonator having an affecting band beside the first one.
An advantage of the invention is that pass band attenuation variation in a bandpass filter can be reduced with a smaller increase in the basic attenuation than in known structures. Resonators are added in both cases. The difference is explained by the fact that the arrangement according to the invention requires a smaller number of extra resonators and the added resonators have a lower energy content than the resonators of a conventional structure. Another advantage of the invention is that the production costs caused by the additional structure according to the invention are relatively small.